This invention relates to speech processing devices and more particularly to apparatus for processing signals in order to determine the characteristics of an optimum hearing aid for a handicapped person. The apparatus has utility both as a testing device for obtaining characteristics in the design of a hearing aid and as a hearing aid unit, as well.
This invention is the subject matter of a Disclosure Document entitled SPEECH PROCESSOR filed on Feb. 3, 1976 as Disclosure Document No. 046651 by Paul Yanick, Jr., the inventor herein.
A patient with a sensorineural hearing loss typically has a reduction in the comfortable range of loudness and an abnormal loudness growth designated as recruitment. The recruitment is frequency dependent and destroys normal relationships between the acoustical elements of speech and hence, reduces the informational capacity of the impaired ear. Such a patient with a sensorineural hearing loss has considerable difficulty understanding speech when it is amplified by a conventional hearing aid. The affected ear, when compared to the normal ear, tends to react with more sensitivity as loudness is increased over threshold strength.
As indicated above, the abnormal growth of loudness or recruitment is frequency dependent and hence, the frequencies affected destroy the significant structure of speech giving rise to distortion and hence, resulting in a loss of information.
Basically, there exists, in literature, graphs depicting the differences in response of a normal and abnormal ear to intensity levels near and above the threshold. Such graphs have been designated as EL graphs or equal loudness contours.
For examples of such results, reference is had to an article published in the Journal of the American Audiology Society, Vol. 1, No. 5 (1976) entitled EFFECTS OF SIGNAL PROCESSING ON INTELLIGIBILITY OF SPEECH IN NOISE FOR PERSONS WITH SENSORINEURAL HEARING LOSS by Paul Yanick, Jr., the inventor herein.
Implicit in the EL contours is a rapid growth of loudness which increases much faster at high frequencies.
There are many hearing aids existing in the prior art which employ various schemes to provide compensation. Such devices typically utilize automatic volume control, peak clipping and compression limiting. These devices have a linear input/output function until a predetermined threshold level is approached. Once this level is approached, the output level remains constant by reducing the gain of the entire signal. These hearing aids compensate to a certain extent for frequency loss by shaping the frequency response of the aid. They do not however, compensate for amplitude and the abnormal loudness growth.
Such devices amplify the energy present in the stronger vowels to levels which mask or obscure the lower energy consonants. Thus, a user of such a device will reduce the volume control and hence, lose the required gain for the lower energy consonants.
Speech sounds, as is known, have a great dynamic range in amplitude. Vowels are about 12 or 13 decibels (db) greater in intensity than the consonants, with the most powerful vowel occurring 28db above the weakest consonant. Hence, vowels have their greatest acoustic energy in the frequency band about 800Hz, while consonants are found above 1,500Hz.
Most subjects with a sensorineural hearing loss have normal hearing for the louder low frequency vowel sounds and subnormal hearing for the weaker high frequency consonant sounds. This, therefore indicates the problem that a user has with a conventional aid and hence, by reducing the volume control on such an aid, he sacrifices the consonant intelligibility for the sake of comfort.
Another conventional approach employed in the prior art is referred to as automatic, non-linear amplitude compression gain control. In these systems, the gain is decreased as the input levels increase. The input/output function is non-linear and is determined by the compression ratio of the particular aid. This method has been investigated and compared to automatic volume control hearing aids and has been described in a paper entitled IMPROVEMENT IN SPEECH DISCRIMINATION OF COMPRESSION VERSUS LINEAR AMPLIFICATION published in the Journal of Auditory Research (1973) in Vol. 13, pages 333-338 written by Paul Yanick, Jr. This technique does provide the hearing aid user with relief from discomfort and renders improved discrimination to the user in relatively low level noise areas. However, in the presence of background noise, the system does not provide the user with adequate relief. The appropriate data and information concerning such systems have been described in an article entitled DISCRIMINATION IN THE PRESENCE OF COMPETITION WITH AN AVC VERSUS A DRC HEARING AID published in the Journal of the American Audiology Society (1973) Vol. 1, No. 4, pages 12-17 by Paul Yanick, Jr.
A user having a sensorineural hearing loss is affected by a decrease in efficiency in adverse listening conditions due to an overmasking which is demonstrated by a shift in the signal to noise ratio and a change in the intelligibility function. Conventional hearing aids greatly increase the disruptive effect of background noise with the signal to noise ratio being 20 to 30db more adverse for the hearing aid user.
It has therefore been determined that in designing a hearing aid, one must consider both the loudness function and the user's ability to communicate in the presence of background noise. As indicated, a great disadvantage of automatic gain reduction in a hearing aid resides in the fact that as the gain is decreased by the stronger vowels and by low frequency background noise, an abnormal relationship between speech sounds occurs. This is due to the fact that the softer consonants are not properly amplified, resulting in a substantial loss in intelligibility. Apart from this fact is the fact that the gain reduction serves to equalize input levels and hence, the signal to noise ratio suffers. These factors effect the user's ability to understand speech by reducing the overall intelligibility.
In an attempt to overcome the deficiency of such a prior art system, a two channel automatic compression gain has been specified. With this technique, the vowels and low level ambient noise do not cause gain reduction, which is necessary for the softer consonants and thus attempt to maintain a normal relationship between speech sounds in such systems. The compression ratio can be set to match the recruitment present at each frequency to compensate for abnormal loudness function.
Such automatic compressor gain systems have been described in an article entitled SIGNAL PROCESSING TO IMPROVE SPEECH INTELLIGIBILITY IN PRECEPTIVE DEAFNESS published in the Journal of the Acoustical Society of America (1973) Vol. 53, No. 6 pages 1646-1657 by Edgar Villchur. This technique provides the hearing aid user with an increase in intelligibility. However, this system suffers in that when the user is in a non-communicative situation, low level ambient noise may become distracting and annoying. In addition, many patients with sensorineural hearing loss do not need any compensation for the abnormal loudness function in the low channel of compression. Persons with ski slope sensorineural hearing loss have normal or near normal loudness function, usually up to 1,500Hz.
It is therefore the object of this invention to provide a hearing aid which avoids the above noted problems by incorporating a compression threshold which is frequency sensitive and adjustable according to the loudness function of the impaired ear. In addition, a 1:1 linear function can be set in the low channel of compression if the loudness function is normal. Hence, a compression ratio of 2:1 may be used in the high channel, while a linear 1:1 mode can be used in the low channel. Post-frequency equalization can then shape the person's equal loudness contours to normal values. In addition, an expansion mode can be set below the compression threshold in the low channel to reduce the low level ambient noise.